Connect to Postgres as SystemD service from docker-compose

Question

I need to connect to a Postgres server instance that is running as a SystemD service from within a docker-compose file.

docker-compose containers ---> postgres as systemd

This is about setting up Airflow with an external Postgres DB that is on localhost.

I've taken the docker-compose example with:

curl -LfO 'https://airflow.apache.org/docs/apache-airflow/2.2.3/docker-compose.yaml'

However in there they are defining a Postgres container where Airflow connects to by resolving the postgres host within the Docker network.

But I already have Postgres running on the machine via SystemD, I can check its status with:

# make sure the service is up and running
systemctl list-units --type=service | grep postgres.*12
# check the process
ps aux | grep postgres.*12.*config_file
# check the service details
systemctl status [email protected]

AFAIU inside the docker-compose YAML file I need to use the feature host.docker.internal so the Docker service makes the docker container find their way out of the Docker network and find localhost with the SystemD services e.g. Postgres.

I've setup the Airflow YAML file for docker-compose with:

---
version: '3'
x-airflow-common:
  &airflow-common
  image: ${AIRFLOW_IMAGE_NAME:-apache/airflow:2.2.3}
  environment:
    &airflow-common-env
    AIRFLOW__CORE__EXECUTOR: LocalExecutor
    AIRFLOW__CORE__SQL_ALCHEMY_CONN: postgresql+psycopg2://airflow:[email protected]/airflow
    ...
  extra_hosts:
    - "host.docker.internal:host-gateway"

There's a lot of stuff going on there, but the point is that the SQLAlchemy connection string is using host.docker.internal as host.

Now when I invoke the command docker-compose -f airflow-local.yaml up airflow-init, then I see in the ouput logs that Airflow is complaining it does not find the Postgres server:

airflow-init_1       | psycopg2.OperationalError: connection to server at "host.docker.internal" (172.17.0.1), port 5432 failed: Connection refused
airflow-init_1       |  Is the server running on that host and accepting TCP/IP connections?

It might be an issue with the DNS resolution between the Docker special network and the OS network, not sure how to troubleshoot this.

How do I make Docker container to find out SystemD services that serve on localhost?